This is a fairly sane description of some of the issues around the BMI measure.

https://www.medicalnewstoday.com/articles/255712

It gets a lot of hysterical rant from pro-fatness zealots, using much the same sort of spin doctoring as the anti-vaxx gang. Historically it was developed as a simple and easy to calculate formula that correlated with empirical risk. It works reasonably well, and there is a very large volume of data using it - an improved measure like the one suggested in that article might well work better for some purposes, but you'd still want to use that old data. And the pro-fatness zealots don't want there to be ANY measure that might describe the damage to individual and public health caused by obesity.